1. Field of the Invention
The present invention relates to a variable gain amplifier for a high frequency band, and more specifically, to a variable gain amplifier for a high frequency band using a microstrip ring hybrid so that the constant matching conditions in input and output may be obtained irrespective of its gain.
2. Description of the Prior Art
Generally, a conventional variable gain amplifier is able to control its gain by converting transmission paths of signals with a switch or by controlling a bias current or voltage of the amplifier.
FIG. 1 is a circuit diagram illustrating a conventional variable gain amplifier with a switch. The conventional variable gain amplifier comprises a fixed variable gain amplifier 101 and a switch 102 which is connected between a input terminal IN and an output terminal OUT of the fixed variable gain amplifier 101.
When the switch is turned OFF, the input IN and the output OUT are disconnected, so that high output power can be obtained by operation of the fixed variable gain amplifier 101. When the switch is turned ON, the input IN and the output OUT are connected, so that low output power can be obtained.
In the conventional variable gain amplifier described above, the low and high output powers can be obtained in accordance with change of the impedance of the amplifier associated with the switch being turned ON and OFF. Most of the variable gain amplifiers obtain high output power with the switch being turned OFF. However, in order to obtain low output power of the variable gain amplifier when the switch is turned ON, the gain of the variable gain amplifier may be lowered due to the resistance of the switch itself. Therefore, it is a problem that good condition of impedance matching can not be obtained with the conventional variable gain amplifiers.